A fast pyrolysis has as its purpose the conversion of carbon-containing feedstock, such as biomass, into highly liquid pyrolysis condensate (pyrolysis oil), as well as into not very solid pyrolysis coke and pyrolysis gas. In the case of the aforementioned biomass, a fast pyrolysis is performed under exclusion of oxygen within a few seconds, preferably in about one second, at approximately 400 to 600° C., preferably at about 500° C., a bio-oil content of 40 to 80% by weight and a biocoke content of only 10 to 30% by weight being reached.
Thus, the fast pyrolysis—also known as flash pyrolysis—is a special pyrolysis method in which a considerable amount of liquid pyrolysis condensate and little gas and coke are formed. In particular, wood and straw (lignocellulose) are able to be liquefied to over 50 to 80% into bio-oil.
The pyrolysis gas is typically separated off from the other two pyrolysis products, the pyrolysis coke and the pyrolysis condensate, and is thus usable as fuel for the aforementioned pyrolysis process.
The remaining liquid pyrolysis condensate and the pyrolysis coke are fed as a mixture of these constituents that forms an oil sludge (bio-oil sludge, slurry) from the fast pyrolysis, to an entrained-flow gasification, where the mentioned products are atomized and gasified in a hypostoichiometric oxygen stream.
By employing entrained-flow gasification at high temperatures and pressures, virtually tar- and methane-free raw synthesis gas is able to be produced at a high conversion efficiency which, above all, is advantageous in a subsequent synthesis. This cannot be accomplished in fixed-bed or fluidized-bed reactors, due primarily to the lower operating temperature. However, when working with an entrained-flow gasification, substantial outlay is entailed in preparing the fuels. Biomass, in particular lignocellulose, such as wood and straw, is able to be most readily converted by fast pyrolysis into a pumpable liquid or a slurry that is pumped using simple pumps into an entrained-flow pressure gasifier and atomized and gasified using oxygen.
However, the known method exhibits a few decisive limitations which complicate or substantially restrict the application, or necessitate special precautions.
Most notably, in terms of its composition, a slurry is often not stable or storable, i.e., following a storage or transport time of greater or lesser length, a segregation, increase in viscosity or other property variations are to be expected. On the one hand, the pyrolysis coke content can settle out in the slurry; on the other hand, pyrolysis condensate contents can separate, an aqueous and an organic phase thereby forming. The aqueous phase may contain greater or lesser quantities of water-soluble organic compounds, in particular acetic acid, alcohols and other hydrocarbons having oxygen or other heteroatoms.
What is dangerous in the case of gasification under oxygen in an entrained-flow pressure gasifier is, in particular, a locally heated aqueous component in the slurry, i.e., an aqueous phase of a mainly low calorific value (for example, low temperature carbonization wastewater) having only small fractions of organic components (for example, alcohols) and pyrolysis coke. Namely, if the aqueous phase exhibits a low calorific value due to a segregation that occurred, i.e., if it contains barely combustible compounds in dissolved or dispersed form, this results in an excess of oxygen in the entrained-flow pressure gasifier that can mix with previously produced synthetic gas and lead to an explosion.
Until now, the dangers and limitations referred to have made it difficult to carry out an entrained-flow pressure gasification of biomass on a large scale. In particular, the aforementioned segregations greatly restrict the capability to transport the intermediate product, the slurry, over relatively long distances, for example, from a decentralized pyrolysis, which preferably takes place directly at the producer of the biomass, to a central entrained-flow pressure gasifier for producing biosynthesis gas. In addition, many slurries produced from seasonal waste material from farming and logging are only storable for a limited period of time in a closed container such as a tank.
When pyrolysis condensates are stored in a tank, even in some cases for relatively long periods of time, there is the risk, in particular, of a phase separation occurring in the pyrolysis condensate between an aqueous phase of low calorific value and an organic phase of high calorific value. The risk arises, in particular, when the tank contents are not able to be thoroughly mixed continuously and with adequate efficiency.
Also, volatizable, low-boiling constituents can alter the composition of the organic and aqueous phase, depending on the type of storage, for one and the same pyrolysis condensate over the course of a storage time.